In electrodeposition coating, as is well known, electrodes are placed in an electrodeposition bath filled with a paint solution. Usually, the electrodes are arranged on both sides of the electrodeposition bath. Then, an object to be coated which is moved between the electrodes arranged in the electrodeposition bath serves as a counter electrode so that an electrically-charged paint resin contained in the paint solution is deposited on the surface of the substrate.
Such electrodeposition coating includes one using a cationic paint whose resin component is positively charged and one using an anionic paint whose resin component is negatively charged. The former is called cationic electrodeposition coating, and the latter is called anionic electrodeposition coating. The cationic electrodeposition coating has been actively studied as a method for base coating for protecting car bodies from corrosion, and has been already used in industry.
Among paints to be used for such electrodeposition coating, a paint obtained by, for example, subjecting a resin having a molecular weight of 2000 to substitution with carboxyl groups to be water soluble is generally used as an anionic paint, and a paint obtained by subjecting its resin component to substitution with amino groups to be water soluble is generally used as a cationic paint. These paint resins exhibit a very low degree of ionization when dissolved in water. Therefore, the conductivity of an anionic paint in water is usually increased by adding a basic electrolyte (electrode solution) such as triethylamine, and the conductivity of a cationic paint in water is increased by adding an acidic electrolyte (electrode solution) such as acetic acid.
However, in a case where the conductivity of a paint solution is increased by adding an electrolyte (electrode solution), a paint resin component contained in the paint solution is reduced with the progress of electrodeposition coating treatment to the substrate. As a result, the concentration of amine, acetic acid or the like as an electrolyte (electrode solution) in the paint solution is increased so that there is a fear that problems such as remelting of a coating film and the occurrence of pinholes arise.
In order to solve such problems caused by an increased concentration of an electrolyte (electrode solution), an electrode device in which a tubular member for supporting a barrier membrane is concentrically arranged around the tubular electrode for electrodeposition coating at predetermined intervals, and at the same time, a barrier membrane, such as an ion exchange membrane, is wrapped around the exterior surface of the barrier membrane supporting member, and water is supplied into an annular gap formed between the electrode and the barrier membrane supporting member through the inside of the electrode to selectively introduce an electrolyte (electrode solution) present outside of the barrier membrane into the annular gap and discharge the electrolyte to the outside is disclosed in Patent Documents 1 and 2.
Patent Document 1: Japanese Patent Application Laid-open No. 5-195293
Patent Document 2: Japanese Patent Application Laid-open No. 2002-60997
By providing the barrier membrane in a tubular manner outside the tubular electrode, it is possible to avoid an increase in concentration of an electrolyte (electrode solution) caused by consumption of a paint resin contained in a paint solution, thereby eliminating various problems such as remelting of a coating film and the occurrence of pinholes caused by the increased concentration of an electrolyte. However, on the other hand, since the electrode device disclosed in Patent Documents 1 and 2 has a double structure in which the barrier membrane and the barrier membrane supporting member are arranged outside the tubular electrode at intervals, it is impossible to avoid the size of the electrode device becoming larger compared to that of the tubular electrode which is an electrode main body.
Further, since the electrode device needs the barrier membrane and the barrier membrane supporting member in addition to the tubular electrode which is an electrode main body, the number of components is increased, thereby making it impossible to avoid an increase in production cost.
Further, as another problem, there is a case where the barrier membrane, such as an ion exchange membrane, arranged outside the tubular electrode at intervals swells or extends in use. For this reason, there is a problem that wrinkles occur in the barrier membrane or it is impossible to firmly fix the barrier membrane to suppress the occurrence of wrinkles. It was a problem that the occurrence of wrinkles in the barrier membrane is a cause of retaining of a resin component contained in a paint solution in the wrinkles, which further causes coating defects such as pits and lumps.
In addition, as an electrode material for electrodeposition coating, an insoluble material in which platinum metal oxides or the like are supported to a valve metal, such as stainless steel, ferrite, or titanium is used. In the case of cationic electrodeposition coating, since an acidic electrode solution such as acetic acid, lactic acid, or formic acid is used as an electrolyte (electrode solution) contained in a paint solution, when using a stainless steel electrode as an electrode, the stainless steel slowly dissolves. As a result, it caused a problem that the electrode solution and an ion exchange membrane are contaminated or it is difficult to reuse the electrode solution. On the other hand, a ferrite electrode is brittle and therefore there was a problem that careful handling was required.